He's still seeing the ball great (OBP) and hitting for average fairly well. His power is all that's missing. Why?
He's hitting more ground balls and getting less line drives.
Notice that he did this back in 2005. The key for him that year was getting the line drive rates back to his career levels. What are his line drives doing this year? Just take a look at the last few games. Career norms. And his fly ball rate is actually better than it was that year.
If you don't recall his 2005 season, check out what his ISO did that year:
Not too shabby of a second half if you ask me. I say buy low.